The present invention relates to the capture of digital images by means of a sensor array in general, and in particular to techniques for preventing blur in the captured image caused by movement of the subject during the digital exposure.
Digital photography appears likely to gain considerable market share from conventional 35 mm photography. This is particularly true for the nonprofessional photographer taking snapshots of family, friends, and vacations. The resolution of digital cameras is rapidly approaching the effective resolution of conventional 35 mm snapshots. Digital photography provides the convenience of being able to preview pictures immediately after they are taken, combined with the ability to email the images, or post the images on a web page.
Digital photography is also finding increasing use in business and commerce. Instantaneous turnaround and the simplicity with which digital images can be incorporated into electronic documents, as well as software used for producing newspapers and trade magazines, means that many weekly newspapers, real estate agents, catalog companies, and other businesses, are moving to digital photography to record images.
Conceptually, the digital camera hag many similarities to the camera used in conventional film photography. A camera lens projects an image onto a sensor array which replaces the conventional light sensitive photographic film. The camera lens may use conventional focusing techniques. An aperture stop maybe used to adjust the focal length ratio or f number of the lens. A shutter is used to control the length of exposure, and a flash may be used to replace or supplement ambient light. All these components are similar to those used in a conventional film-based camera, the differences being related to the particular attributes unique to the digital sensor which records the image.
Both digital photography and conventional film-based photography sense and record information about the relative number of photons which each part of the image receives from the actual scene being imaged by way of the lens. Conventional film-based photography employs a light-sensitive chemical, typically a silver halide, which is chemically activated by the absorption of several photons of light. Silver halide crystals which have been activated are then developed with chemistry to form an image. The more light that strikes a particular portion of the film, the more halide crystals which are activated, the darker the resulting image upon development.
The resolution of film-based photography depends on the size of the individual silver halide grains which form the emulsion deposited on the surface of the film. There is typically a trade-off between film resolution and film sensitivity to light. Greater sensitivity to light is advantageous because a shorter exposure can be used to form an image. Modem photographic emulsions are relatively efficient detecting perhaps 20 to 30 percent of the photons which impact the film.
For conventional film-based cameras, image quality depends on film selection, the mechanical and optical qualities of the camera and properly focusing the image on the film plane and properly timing the exposure.
Digital photography relies on a sensor array, with each sensor within the array corresponding to the minimum resolution element, or pixel in the final image. A digital camera typically has one million to a few million sensors arranged in a rectangular array. The sensor arrays are microelectronic devices formed on a silicon-based chip, or other semiconductor substrate. The devices are built utilizing conventional semiconductor device fabrication techniques, such as vapor deposition, ion implantation, application of photoresist followed by etching, etc. A typical sensor is a semiconductor device which captures electrical charge within a potential well, the electrical charge being proportional to the number of photons absorbed by the semiconductor device.
After the sensor array has been exposed to light, the amount of electrical charge in each sensor potential well within the array is measured and converted by an A/D converter to a digital value. The semiconductor devices making up a digital array have quantum efficiencies, i.e. the percentage photons detected, which may be as high as around 60 percent. Unlike photographic emulsion, where each individual silver halide crystal is either activated or not, each semiconductor sensor within the array can have between 0 and several thousand electron pairs. Thus each sensor within the array has a much larger dynamic range than the crystals making up a photographic emulsion, which are basically bipolar.
An approximation of the ideal exposure for forming an image on any sensor array is an exposure in which the brightest part of the image just saturates the array sensors. In this way the fall dynamic range of the sensor array is utilized. However, because the dynamic range of the typical semiconductor sensor array is so much greater than film, an acceptable image can be acquired with an exposure which only utilizes a fraction of the dynamic range of the sensors. Nevertheless, in conventional digital photography it is desirable to use the full dynamic range of the sensor array both to allow later adjustments in dynamic range and to overcome the problem of noise. Extraneous electrical charge which accumulates within individual sensors due to thermal noise and other factors within the microelectronic devices is more of a problem with a sensor which has accumulated only a small number of photons.
Beyond obtaining a digital image, the digital camera still functions much like a conventional film-based camera in capturing an image. What is needed is a digital camera which provides new functionality based on the inherent properties which the new technology brings to image capture.
The digital camera of this invention utilizes a sensor array which is capable of removing blurring from an image caused by motion of the imaged subject. This is accomplished by monitoring the rate at which charge is collected by the individual sensors within the array while a scene is being imaged. If the rate at which photons are striking the sensor varies, then the brightness of the image which the sensor is viewing must be changing. Changing image brightness implies that the object being imaged is moving. When a circuit built into the sensor array detects that the image brightness is changing, the amount of charge collected is preserved, and the time at which motion was detected is recorded. The amount of charge collected may be preserved by closing an optical shutter such as a liquid crystal shutter in front of a particular sensor, or by transferring the charge to a storage location. When an exposure is complete, portions of the image where no motion was detected have a normal dynamic range of image intensity. Those portions of the image where motion was detectedxe2x80x94and thus where exposure was stopped before exposure was completexe2x80x94have a reduced dynamic range. Each pixel corresponding to each sensor where exposure was stopped is adjusted to the proper exposure by linearly extrapolating the exposure the sensor did receive, so that the pixel corresponding to that sensor has an intensity corresponding to the dynamic range of the entire image. Thus, for example, with a particular sensor which was exposed for only one-tenth of the total exposure time before motion was detected, the value corresponding to the sensor will be multiplied by a factor of ten.
The effect of motion within a scene being imaged with a conventional digital or film-based camera is blurring of the object which moved and the obscuring of that portion of the image over which the moving object passes. With a digital camera of this invention the effect of motion of the object during an exposure is simply to reduce the dynamic range and to increase slightly the noise associated with the object which moved and a portion of the image which was obscured by the moving object. The effect of decreased dynamic range and increased image noise are much less apparent than blur caused by motion.
The image sensing array will preferably be implemented in CMOS as active-pixel sensors. The CMOS technology provides lower cost, and allows cost-effective implementation of additional on-chip circuitry necessary to implement the motion detection, and blur elimination functions.
It is an object of the present invention to provide a digital camera which removes blur caused by motion of the subject.
It is another object of the present invention to provide a digital camera which, during a single exposure can capture both the blurred and the preblurred image.
It is a further object of the present invention to provide a digital camera with reduced blur under low light conditions.
It is a yet further object of the present invention to provide a low-cost digital sensor for digital cameras which incorporates subject motion blur prevention.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.